This invention relates to the separation and purification of ruthenium from its solutions containing other platinum group metals and base metals.
In this specification the term "base metal" refers to any metal other than a member of the platinum group of metals and gold.
Ruthenium is one of the platinum group metals and is usually recovered with the other platinum group metals in the form of a platinum group metals concentrate, wherein the ruthenium concentration is relatively low.
Conventional techniques for the recovery and purification of ruthenium are based on the formation of an oxide which boils at a low temperature relative to many other oxides. Thus ruthenium and osmium tetroxides can be formed by oxidation of the metals in solution, and then removed from solution by boiling.
For osmium, oxidation of the metal to the VIII oxidation state is relatively easy, and a number of oxidising agents can be used. Furthermore, osmium can be efficiently removed as the tetroxide forms even under fairly strongly acid conditions.
For ruthenium however, the oxidation is more difficult and control of the solution pH at a relatively high value is essential. Under these circumstances, removal of ruthenium from solution is incomplete and generally several hundred parts per million of ruthenium remain in the solution.
This not only represents a loss in ruthenium recovery, but the ruthenium remaining forms an impurity element in the further processing of the solution for the recovery of the other platinum group metals.
Other disadvantages occur including the contamination of the ruthenium distillate with an acid spray which is often formed and also the highly unstable nature of ruthenium tetroxide makes it dangerous owing to the possibility of an explosion.
Separation of ruthenium using solvent extraction and ion-exchange methods has not generally proved very successful. An exception may be found in the treatment or radio-active waste materials which contain appreciable concentrations of ruthenium. The usual treatment is by solvent extraction from a nitric acid solution. In such solutions ruthenium occurs as the nitrosylruthenium nitrate complexes which can be separated from the solution by solvent extraction with, typically, long chain tertiary amines.
It is well known that ruthenium forms a very large number of nitrosylruthenium complexes and that the stability of such complexes is greater for ruthenium than for any other element. Thus, for example, in hydrochloric acid solution the nitrosylruthenium complex RuNOCl.sup.2-.sub.5 can be formed. This complex is highly extractable, and, by forming such a complex preferentially, separation of ruthenium from the other platinum group metals may be achieved. The use of such a process has, however, up to now, been hampered by the following two factors:
1. the available methods of making the nitrosylruthenium complex often have the disadvantage that the reaction yield is not 100%, but typically 90-95%, or, when a quantitative yield is obtainable such available methods are very inconvenient to implement in practice; and, PA1 2. during the course of the reaction the other platinum group metals in solution tend to form complexes, although of a different nature, which behave towards anionic solvent extractants in a manner similar to the nitrosylruthenium complex. PA1 conversion of the ruthenium to a nitrosylruthenium complex with the ruthenium in the Ru.sup.+2 oxidation state; conversion of the nitrosylruthenium complex to a nitrosylruthenium chlorocomplex and any other platinum group metals to chlorocomplexes; and, PA1 the removal of the nitrosylruthenium chlorocomplex from solution using a suitable liquid or resin anion exchanger. PA1 removing any solids by filtration; PA1 reducing the ruthenate anion to the insoluble dioxide using a suitable alcohol, such as ethyl alcohol; PA1 removing the insoluble dioxide from solution by filtration; and PA1 removing entrained impurities from the ruthenium dioxide using a suitable wash and then reducing the ruthenium dioxide to ruthenium metal. PA1 1. A solution, from which gold, platinum and palladium have been removed, is concentrated and rediluted with water to give a free hydrochloric acid concentration of about 0.5M. If the separation of platinum and palladium has been achieved using the amino acid solvent as described in our co-pending patent application Ser. No. 806,816, entitled SEPARATION AND PURIFICATION OF PLATINUM AND PALLADIUM this will already have been achieved. PA1 2. If osmium is present in solution, it is removed by adding nitric acid and boiling the solution to remove osmium tetroxide. The distilled osmium tetroxide may be condensed and recovered by conventional techniques e.g. reduction with alcoholic sodium hydroxide, followed by acidification with hydrochloric acid and finally precipitation of (NH.sub.4).sub.2 OsCl.sub.6 by addition of ammonium chloride. PA1 3. Once the osmium has been removed from solution the ruthenium may be converted to the nitrosylruthenium complex as follows: PA1 4. At this stage the ruthenium will be totally converted to nitrosylruthenium, but a series of such complexes will exist probably consisting of mixed aquo-chloronitro species. Inevitably the rhodium and iridium will also be present as a number of mixed complexes and iridium additionally will be present mainly in Ir.sup.IV oxidation state. The solution must be treated further to destroy these complexes. It has been found that by boiling the solution after acidification with HCl, preferably to at least 6N, the nitrosylruthenium and the rhodium and iridium may be converted to the chlorocomplexes. Additionally, a reductant capable of reducing Ir.sup.IV to Ir.sup.III may be added. SO.sub.2 is a convenient reductant for this purpose. PA1 (i) stripping is complete in one contact, and the process is thus very easy to operate. PA1 (ii) the strip solution so obtained lends itself to simple processing for final purification and precipitation of pure ruthenium dioxide. PA1 a. the hot strip solution is treated with sodium bromate to convert the nitrosylruthenium complex to the soluble ruthenate anion RuO.sub.4.sup.2-. The oxidation process renders insoluble most base metals and other platinum metals, and these may be removed from the solution by filtration. PA1 b. the purified solution is then treated with alcohol in sufficient quantity to reduce the ruthenium to the insoluble dioxide, which is recovered by filtration. PA1 c. the ruthenium dioxide is washed with dilute acid to free it of entrained impurities, mainly sodium, and then reduced with hydrogen at an elevated temperature to produce pure ruthenium metal. PA1 1. the loaded resin is washed free of excess with water; PA1 2. a saturated solution of SO.sub.2 is passed through the resin. The pink colour of the resin beads fades as reaction occurs with the formation of the sulphito complexes of RuNo,Rh(111) and Ir(111); PA1 3. these complexes may now be effectively removed from the resin by elution with hydrochloric acid, preferably in the concentration range 2M-6M.
It is an object of this invention to provide a process for the separation and purification of ruthenium as a nitrosylruthenium complex which provides for a high yield of this complex and the formation of the other platinum group metals present into forms suitably different in their behaviour in comparison with the nitrosylruthenium complex.